I have a video to see the problem ( Hold items and open inventory, put items in offhand and close inventory, again we open inventory. Then there will be a copy of the items )
https://youtu.be/AoqxB6ZvrQA
I use paper spigot: 1.10.2 & OCM latest

So what exactly are you doing clicking-wise? Are you closing the inventory immediately after clicking into the offhand slot?

After I clicked the item and put in offhand. I immediately close inventory

Another way, you can try clicking constantly a items put in offhand and immediately close inventory. It will produce the same result as in the video.

@tantanpro1612 I can reproduce it, but the duplication is purely client side. Can you drop the items and pick them back up or relog?